Use assert as in the rest of the BN library.

crypto/bn/bn_mul.c

@@ -56,6 +56,11 @@
  * [including the GNU Public Licence.]
  */
 
+#ifndef BN_DEBUG
+# undef NDEBUG /* avoid conflicting definitions */
+# define NDEBUG
+#endif
+
 #include <stdio.h>
 #include <assert.h>
 #include "cryptlib.h"